Pressure measuring transducers are used for pressure measurement in almost all branches of industry. Measured pressure values are used e.g. for control (open- or closed-loop control) and/or for monitoring of industrial manufacturing- and/or treatment-processes.
Popular in pressure measuring technology are so-called semiconductor-sensors, e.g. silicon-chips with doped resistor regions, which serve as pressure-sensitive elements. Usually, such pressure sensors utilize a pressure sensor chip in the form of a membrane, or diaphragm, installed in a pressure measuring chamber. Pressure sensor chips are, as a rule, very sensitive and are therefore not directly subjected to the medium, whose pressure is to be recorded. Instead, a pressure transfer means filled with a liquid and having an outer, separating diaphragm is interposed. In operation, the pressure to be measured is applied to the separating diaphragm and transmitted via the pressure transfer means to the pressure sensor chip located in the pressure measuring chamber. This is the situation for measuring absolute pressures, as well as also for measuring relative-pressures and pressure differences.
The inner volume of pressure transfer means and measuring chamber is sealed to liquid in properly operating measuring transducers. There is, however, a danger, that the pressure measuring transducer becomes damaged in such a manner, that liquid escapes from this inner volume. This can come about e.g. as a result of mechanical damage to the separating diaphragm or from unsealed places in the region of the securement of the separating diaphragm, at the filling opening, via which the liquid was filled, or in the region of feed-throughs, e.g. for electrical connection lines of the pressure sensor. Liquid loss leads to change of the measuring characteristics of the measuring transducer. Depending on how large the liquid loss is, such can lead to a reduction in the accuracy of measurement, to considerable functional disturbances or, in the worst case, to a total failure of the measuring transducer. Significant functional disturbances occur especially in the case of low temperatures and/or in the case of high pressures acting on the pressure measuring transducer.
Additionally, the emerging liquid can cause damage, when it leaks, e.g. through a crack in the separating diaphragm, into an industrial manufacturing process, such that it gets mixed into processed products.
In German Patent DE-A 10 2004 019 222, apparatus and method are described for diagnosis of liquid losses from pressure measuring transducers filled with pressure transmitting liquids, wherein a test element is arranged in the pressure measuring chamber of the pressure measuring transducer. For diagnostic purposes, the volume of the test element can be increased by appropriate operation thereof. An increasing of the volume leads to a pressure rise in the pressure measuring chamber, which is then registered by means of the pressure sensor. A diagnosis of leakage is accomplished by registering, as a function of time, pressure rise brought about by a volume increase and comparing the resulting functional relationship with a corresponding reference curve of the intact pressure measuring transducer. If a liquid loss is present, the maximum pressure rise brought about by a defined volume increase is smaller than would be the case, if the pressure measuring transducer were intact. Additionally, liquid of the pressure measuring transducer can be pressed out by the volume increase. This leads then, for example, to a time-delayed falling of the pressure in the pressure measuring chamber, which is reflected in the characteristic curve recorded during the diagnosis.
This method has the disadvantage that the measurement effect achievable by the volume increase of the test element is only relatively small. The reason for this lies in the fact that separating diaphragms are, as a rule, as thin as possible and have a spring stiffness, which is as low as possible, in order to achieve a best possible, uncorrupted transmission of the external pressure acting on the separating diaphragm. Leakages only get detected in this way, after a relatively large amount of liquid has already escaped. This has the result that, as a rule, only major damage to the separating diaphragm can be detected, while smaller leakage locations, e.g. in the region of the separating diaphragm, on other mechanical components or at joints, where only low liquid losses arise, remain undetected.
A further disadvantage lies therein, that the pressure in the pressure measuring chamber is increased during the diagnosis. Through such pressure increase in the presence of a leak, under the right circumstances, even more liquid can be expelled out of the pressure measuring transducer.